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A library for spelling-correction based on Levenshtein Automata.
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package com.github.liblevenshtein.transducer;
import com.github.liblevenshtein.collection.AbstractIterator;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Iterator;
/**
*
* Lazy implementation of {@link Iterable}, which in subsequent invocations of
* its {@link #iterator()} searches only enough of the dictionary automaton to
* find the next spelling candidate. Subsequent calls to {@link Iterator#next()}
* pick up where the search stopped.
*
*
*
* Please note that the {@link #iterator()} is not currently threadsafe, so if
* you will be consuming the same {@link Iterator} from multiple threads you
* will need to synchronize access yourself.
*
*
*
* The algorithm for imitating Levenshtein automata was taken from the
* following journal article:
*
*
*
*
* {@literal @}ARTICLE {Schulz02faststring,
* author = {Klaus Schulz and Stoyan Mihov},
* title = {Fast String Correction with Levenshtein-Automata},
* journal = {INTERNATIONAL JOURNAL OF DOCUMENT ANALYSIS AND RECOGNITION},
* year = {2002},
* volume = {5},
* pages = {67--85}
* }
*
*
*
*
* As well, this Master Thesis helped me understand its concepts:
*
*
*
* - www.fmi.uni-sofia.bg/fmi/logic/theses/mitankin-en.pdf
*
*
*
* The supervisor of the student who submitted the thesis was one of the authors
* of the journal article, above.
*
*
* @author Dylon Edwards
* @param Kind of nodes of the dictionary automaton.
* @param Kind of the spelling candidates returned from the
* dictionary.
* @since 2.1.2
*/
public class LazyTransducerCollection
extends AbstractIterator
implements Iterable {
/**
* Query term whose spelling should be corrected.
*/
private final String term;
/**
* Maximum number of spelling errors candidates may have from the query term.
*/
private final int maxDistance;
/**
* Attributes required for this transducer to search the dictionary.
*/
private final TransducerAttributes attributes;
/**
* Breadth-first traversal of the dictionary automaton.
*/
private final Deque> pendingQueue = new ArrayDeque>();
/**
* Transitions one state to another.
*/
private final StateTransitionFunction stateTransition;
/**
* Helper variable used when determining the length of a characteristic
* vector.
*/
private final int a;
/**
* Length of the next characteristic vector to return.
*/
private int k;
/**
* Offset of where to begin traversing the next characteristic vector.
*/
private int i;
/**
* Labels of the outgoing transitions for a dictionary state.
*/
private Iterator labels = null;
/**
* Current intersection between the dictionary and Levenshtein automata.
*/
private Intersection intersection = null;
/**
* Initializes a new LazyTransducerCollection with a query against the
* dictionary automaton.
* @param term Query term whose spelling should be corrected.
* @param maxDistance Maximum number of spelling errors candidates may have
* from the query term.
* @param attributes Attributes required for this transducer to search the
* dictionary.
*/
public LazyTransducerCollection(
final String term,
final int maxDistance,
final TransducerAttributes attributes) {
this.term = term;
this.maxDistance = maxDistance;
this.attributes = attributes;
pendingQueue.addLast(
new Intersection(
attributes.dictionaryRoot(),
attributes.initialState()));
this.stateTransition =
attributes.stateTransitionFactory().build(maxDistance, term.length());
// f(x) := x * 2 + 1
// a := (n - 1) / 2
// f(a) = (n - 1) / 2 * 2 + 1 = n - 1 + 1 = n
//
// We want to cap the value of "a" at "n = max integer" so "f(a)" does not
// overflow, which it would if "a > (n - 1) / 2". In other words, define:
// g(x) := { x * 2 + 1 , if a < (n - 1) / 2
// { n , otherwise
//
// If there was no upper bound for integer values, this would be equivalent:
// h(x) := min {f(x), n}
this.a = maxDistance < (Integer.MAX_VALUE - 1) >> 1
? (maxDistance << 1) + 1
: Integer.MAX_VALUE;
}
/**
* {@inheritDoc}
*/
@Override
public Iterator iterator() {
return this;
}
/**
* {@inheritDoc}
*/
@Override
protected void advance() {
while (null == next
&& (null != labels && labels.hasNext() || !pendingQueue.isEmpty())) {
if (null != labels && labels.hasNext()) {
final DictionaryNode dictionaryNode = intersection.dictionaryNode();
final State levenshteinState = intersection.levenshteinState();
final char label = labels.next();
final DictionaryNode nextDictionaryNode =
attributes.dictionaryTransition().of(dictionaryNode, label);
final boolean[] characteristicVector =
characteristicVector(label, term, k, i);
final State nextLevenshteinState =
stateTransition.of(levenshteinState, characteristicVector);
if (null != nextLevenshteinState) {
final Intersection nextIntersection = new Intersection(
intersection,
label,
nextDictionaryNode,
nextLevenshteinState);
pendingQueue.addLast(nextIntersection);
if (attributes.isFinal().at(nextDictionaryNode)) {
final int distance =
attributes.minDistance().at(nextLevenshteinState, term.length());
if (distance <= maxDistance) {
final String nextCandidate = nextIntersection.candidate();
this.next =
attributes.candidateFactory().build(nextCandidate, distance);
}
}
}
}
else {
this.intersection = pendingQueue.removeFirst();
final DictionaryNode dictionaryNode = intersection.dictionaryNode();
final State levenshteinState = intersection.levenshteinState();
this.i = levenshteinState.head().termIndex();
final int b = term.length() - i;
this.k = a < b ? a : b;
this.labels = attributes.dictionaryTransition().of(dictionaryNode);
}
}
}
/**
* Returns the characteristic vector of the term, from its characters between
* index i and index k. The characteristic vector contains true at each index
* where the corresponding character of the term is the value of x, and false
* elsewhere.
* @param x char to find all occurrences of in the relevant substring of term
* @param term Term in which to find all occurrences of the character, x
* @param k Length of the substring of term to examine
* @param i Base-index of the substring of term to examine
* @return Characteristic vector marking where x appears in the relevant
* substring of term.
*/
private boolean[] characteristicVector(
final char x,
final String term,
final int k,
final int i) {
final boolean[] characteristicVector = new boolean[k];
for (int j = 0; j < k; ++j) {
characteristicVector[j] = x == term.charAt(i + j);
}
return characteristicVector;
}
}